Making Noise

When calibrating the Elecraft K2 crystal filters, a noise generator becomes an invaluable tool. Using one results in clean spectrum traces. I had built an RX noise bridge some time ago from an ARRL Handbook -- the original article for that project appeared in the December 1987 issue of QST.

The noise source is a reversed-biased zener diode -- which produces noise of fairly broad bandwidth. A two transistor amplifier chain feeds the trifilar winding of the noise bridge transformer.

This circuit had the unusual addition of an oscillator to switch the noise off and on at about 1 kHz. This made for the very disconcerting effect of hearing 1 kHz tone no matter where the receiver was tuned. The noise bridge was intended to help me tune a manual antenna tuner without having to transmit, but it didn't work as expected.

The oscillator was useless for tuning the filters of the K2. I modified the circuit by removing the NE555 chip and jumpering pin 3 to pin 4, effectively supplying power to the zener all the time. (In fact, this very circuit appeared in the March 2002 issue of QST, and was also promoted by Tom, N0SS as a noise generator on his site)

However, the two transistor amplifier didn't seem to provide much output -- I got the strongest noise signal when attaching to the base of the first transistor. Perhaps this is why the tuner-tuner project didn't work so well. I suspect there's something wrong with one of the junkbox transistors I used.

My main problem was that I needed to jury-rig it with jumpers every time I wanted to use it. What I wanted was a dedicated noise generator -- which isn't much more than a zener diode noise source, followed by a broadband amplifier.

Since I didn't have much luck with the two-transistor amplifier, I thought of using something different. Mark, WA3YNO suggested using an LM703 as the amplifier. I had a better idea -- why not use a MMIC amplifier? The MMIC offers high gain, unconditional stability and an impedance near 50 ohms. I'm surprised we don't see more MMICs in amateur designs. I had a few MSA 0885's in my junkbox. Perfect.

Better engineers than I have already figured this out. If you look at the Elecraft N-gen, this is exactly what they did. So I build basically the same circuit on a piece of perfboard. The result -- S9+20 dB worth of noise. This circuit is so simple it's hard for it not to work right away. A switch is handy because the circuit draws over 12 mA from a 9 volt battery, so it won't last terribly long if you leave it on.

This noise generator works great. Make sure you don't ever transmit into it, though, as just a little bit of power will destroy the MMIC in short order. I generally use the Rx Antenna jack on the K2 (since I have the 160m module), so there's no chance of transmitting into the noise generator. Just make sure you switch the Rx Antenna off on that band, or you'll be wondering where your signals went. (Don't ask me how I know this)

Who Says Ham Radio Is Dying?

OK, I've been watching this for a while now, probably since I stumbled on the site in about 2004. Set up by Joe, AH0A, it tracks statistics on the FCC amateur license database. It has been very interesting to watch.

If you look at the table of total licenses, there are some predictable things happening: Advanced and Novice licenses have been dropping off steadily since they stopped issuing them back in April of 2000. In fact, there are so few of these, I wonder why the FCC doesn't just grandfather them into Extra and Technician class, respectively.

However, the number of Extra, General and Technician class licensees have been going up for some time. After the FCC stopped testing for CW proficiency, the number of Extra class licenses has skyrocketed. Technician and General class licenses have been on the rise, too, if you overlook all the licenses being upgraded to General and Extra.

But the exciting news is that Extra, General and Technician class licenses are at record levels, and the total number of licenses has surpassed it's previous record.

While certainly not all the licensees are active, it is good to see the increasing numbers. A healthy number or licenses should keep amateur radio alive for a long time.

Configuring Elecraft K2 Crystal Filters

The Elecraft K2 has a number of unique features, one is the variable bandpass filter which can be programmed for different bandwidths. This leads to a very flexible design for CW or RTTY, but requires a bit of configuration work. For each mode, you can select up to four crystal filter configurations (FL1-FL4), including using the KSB2 module filter (OP1). The first filter configuration (FL1) is also used when transmitting.

Typical configurations use OP1 in the FL1 position. For CW, I opted to use 1000 Hz in FL1, then with the progression of 400 Hz, 160 Hz and OP1 for FL4. The 1000 Hz setting is about the widest setting that still has smooth sides from a single peak. Much wider, and one side starts to get a pronounced "hump". This filter is good for general tuning on an active band. 400 Hz is great for crowded contest conditions. I rarely use 160 Hz, but it is useful for digging out weak ones. I centered these on 600 Hz. The OP1 filter is good for tuning a quiet band, but it is harder to zero-beat stations.

SSB and RTTY require OP1 in the FL1 position. On SSB, I used 1.8 and 1.6 kHz settings for FL2 and 3, but I rarely use them, the asymmetric filter makes for harsh listening.

RTTY necessitated a minor modification. During the A to B modifications, I added 47 and 100 pF caps across C174 and C173, respectively for the BFO stability mod. I changed these to 56 and 120 pF. However, this did not shift the BFO frequencies at all. I added 3 pF from the X3/X4 junction to ground and this lowered the lower BFO frequency 800 Hz without affecting the upper frequency. The wider BFO frequency range is necessary to accommodate the higher tones use on RTTY.

I originally picked the "low tone" frequencies of 1275 / 1445 Hz for RTTY. However, the resulting center frequency of 1360 Hz, can't be used with the KDSP2 filters. The KDSP2 filters can only be set to multiples of 100 Hz. So, I switched to frequencies of 1415 / 1585 Hz, nicely centered around 1500 Hz.

For the rest of the RTTY filters, I used 1000 Hz, 500 Hz and 300 Hz. 1000 Hz is good for general listening, 500 Hz is pretty much single-signal, and 300 Hz clips the edges of the signal passband, but is useful for trying to dig out stations with surrounding QRM.

Align the filters using the standard procedures for the CW and SSB filters, the RTTY filters are pretty much the same as the CW filters, except the center frequency is different. Instead of Spectrogram, I use CocoaModem on the Mac. If you go to the Config window, CocoaModem has a nice spectrum display. You can set the RTTY frequencies as markers in the window. You can see the picture at the top of the article shows CocoaModem displaying the 400 Hz filter spectrum, with markers at 550 and 650 Hz.

There's two things to know about setting the K2 filters. First, there's a limit to the resolution of the DAC used to tune the BFO -- it may not be possible to get the BFO of each filter exactly on the right frequency. The net result is that the frequency of a desired signal may shift slightly when moving from filter to filter. Second, it's important to match up both sidebands so they look and sound the same. Because of the VCO design, the K2 inverts signals above 20 MHz, so the filter used for LSB is used for USB above 20 MHz and vice versa. (The K2 firmware takes care of this automatically)

When setting up the filters, it's helpful to try to flip between opposite sidebands (using the CW RV button), and check that both sidebands sound about the same. Bumping the BFO setting by one tick can often be helpful in getting them to match.

Similarly, walk through the filter settings (using the XFIL button) to monitor any shift in the received signal. Moving the BFO setting one tick can help in some cases, but because of the DAC resolution it won't be completely perfect. With a little patience, it isn't hard to get the filters as close as possible.

The filters in the K2 offer excellent performance in an inexpensive radio, it just takes a bit of care to configure them.

WARC Trap Dipole

Although I've had good success using an untuned doublet on the WARC bands, I made up my mind that it would be nice to have an antenna that didn't require a tuner.

Initially, I built this antenna has a 30m / 17m dipole. The traps are made from 1.5" Schedule 40 PVC pipe, which is about 1.9" in diameter, wound with RG-59U coax. I built them in much the same manner as those I used later in the 80/40m dipole.

The 17m traps are 35 1/2" of coax wound as 4.8 turns. I trimmed these traps to 18.1 MHz. I actually built three traps, the first was a dummy form so I could figure out where I had to drill the holes to pull the coax down tight.

The 30/17m version uses 12 1/2 feet of wire in each leg, and the outer segments are 8 feet. You may have to trim the lengths to resonate the dipole in the band.

Adding 12m requires two more traps. These were made from 26 1/2" of coax wound in about 3 and 3/4 turns. Trimming the 12m traps right on frequency takes some doing, as small changes in length can shift the frequency of the trap considerably.

I inserted the 12m traps 9 feet 5 inches from the center, and then trimmed the length of the outer legs to bring resonance within the bands. I did not do a great job of measuring -- the 12m traps ended up in slightly different locations in each leg. It does not appear to affect the antenna much.

You should measure the antenna at some height, as the tuning will change as the antenna is raised. I did my initial trimming at 15 feet, and when raised to 42 feet, the resonance frequency rose over 200 kHz on 30m, and slightly less on 17m.

I built a current balun into the center mounting box for the dipole, using 10 type 43 1" ferrite beads, slipped over coax inside the box.

Since the WARC bands as so small, antenna SWR bandwidth isn't a consideration. At 42 feet, there's a little bit of a broadside pattern on 30, more so on 17m and 12m. Since I installed this antenna as an inverted V, the pattern is a bit more circular. The traps can easily handle the 700 or so watts on 17 and 12m.

The WARC bands are a lot of fun with a decent antenna. I've worked several DXpeditions with this dipole on 17m. Without sunspots, there hasn't been a whole lot of activity on 12m yet.

This sort of antenna is easy to build and install. There's no reason lack of antenna should keep you off the WARC bands.

Trap Dipole for 80/40m

At my old QTH I used an 80m untuned doublet: 125 feet fed with open wire using a remote balun. I put it up my at current QTH, shortening it to about 115 feet to fit my lot.

It worked OK for a low dipole, but it was a bit troublesome. Even with a Murch UT-2000A tuner, I had trouble using it with my amplifier. It also took time to retune when switching bands.

I decided what I needed was a trap dipole. I'd had some success building a trap dipole for the WARC bands using coaxial traps. After reading the analysis by W8JI, I still wanted to use coaxial traps, but I decided to use traps resonant off the operating frequency.

You can see the result above, about half the antenna is visible, with the feedpoint on the left, and one of the traps just to the right of center.

The traps are designed to be resonant about 6.6 MHz. They are made of 76 inches of RG-59A/U, wound in 6 and 1/3 turns around a four inch long piece of 3" schedule 20 PVC. The actual frequency of resonance isn't critical, so long as both traps are identical.

The traps are wired in series, as shown below, the inner wire (black) attaches to the center conductor of the coax, and the outer wire (red) attaches to the shield of the opposite end.

The shield of the inner end is connected to the center conductor of the outer end, as diagrammed by VE6YP.

The center section is made of two 33 foot pieces of wire. In retrospect, another foot might have been better. The outer sections started at 26 feet each. I cut off 10 feet 8 inches to bring the antenna into the band, resulting in 15 feet 4 inches. If you were to duplicate this antenna, I would start with 35 feet and 20 feet, respectively, and trim from there.

Fed with RG-8x to a height of 35-42 feet, the antenna resonates at 7.22 MHz with a VSWR of 1.5:1, and 3.76 MHz with 1:1. With the typical automatic antenna tuner in modern rigs, or the pi network of older amplifiers, this antenna easily covers both 40 and 80m. Being so low, this antenna doesn't have much of a pattern on 80m, and only a gentle broadside pattern on 40m.

I've been pretty happy with the dipole. It works well on 80m early in the evening when the propagation is short. Not bad for an antenna that's less than 100 feet long.

I hope to add a couple more traps to add 30 and 20m this summer. I'll let you know how that works out.

My Elecraft K2 Wish List

I've owned my K2, SN #2548, for several years now. It has been my primary radio since the fall of 2002, when I added the KPA100 module to it. I really enjoyed building it, and I'd like to build another. A couple of years ago, when the K3 first came out, Wayne Burdick at Elecraft collected wish lists of K2 features. Given the runaway success of the K3, I have my doubts if they will ever get back to enhancing the K2, but in the spirit of hopefulness, here's my list:

Firmware-only Suggestions:

• RTTY Mark Display Frequency - Just as CW displays the carrier frequency of the received CW signal, the RTTY mode ought to display the frequency of the Mark signal. A menu option would be needed to set the frequency of the Mark signal. (This would essentially subtract or add the Mark frequency from the LSB / USB carrier frequency, respectively)
• VOX Toggle - I'd rather toggle VOX on and off, and have the VOX delay time set in a menu. Having to cycle through all the VOX delay times to turn VOX on and off is a real pain.
• Tuning Rates should follow mode - 10 Hz tuning is fine for CW and RTTY, but too slow for SSB. 50 Hz tuning is fine for SSB. The rate button could then be used to switch to 1 kHz step and back to 10 or 50 Hz. It would save a lot of button presses trying to get the right tuning rate. You could also display all the digits of the frequency regardless of tuning step (in which case SSB would alternate the bottom digit between 0 and 5)
• RIT / XIT Display - momentarily display the RIT or XIT offset on the main display when it is changed (much like for power or keyer speed).
• Unified Filter - Rather than having to separately select XFIL and AFIL settings, I'd rather have three of four filter presets that allow me to combine an XFIL and AFIL setting for each mode. One button (say XFIL) would then cycle through the presets. That way, you're always selecting an optimized filter solution, rather than selecting it manually, which is made more difficult because there is no filter display indicator.
• IF Shift - This very handy feature could be implemented with a firmware change. Biggest problem is how to control it. I find the XIT to be useless, so I would vote to replace XIT with IF Shift. An alternate would be to use the Keyer Speed knob in SSB or RTTY modes.
• CW Tuning Direction - an option to flip sidebands on 15m and above, so that the tuning direction stays the same. (This won't work on RTTY or SSB, of course)
• CW Memory Command - have a computer command to write to the CW memories. That way you can program them from the computer and use them from the front panel.
• INP mode for CW PTT - This mode would allow PTT on the dot line and CW keying on the dash line - great for contesting work. (this would disable auto-detect of hand keying)
• Variable Rate Turning - normal tuning is one step at a time, but if you turn the knob faster, the tuning rate increases for as long as you tune quickly.
• Auto-Tuner Out -- Allow one-touch selection of Tuner AUTO/CAL. This would allow one to easily switch the tuner on and off.

Hardware Improvements:

• Improved KSB2 module -- starting with a filter with sharper skirts (better than 1.5 shape factor), 6-10 dB more mic gain available, more positive VOX (using separate VOX amplifier), anti-VOX circuitry.
• Improved KNB2 module -- allow selection of longer blanking times, more thresholds, better blanking action.
• PFx Module -- a tiny little module that has four (or so) buttons. Hooks into the aux line and offers access to eight functions of the K2 (four on tap, four on hold). Obviates the need for FPLY, opens the K2 up for easier access to things like switching filter presets, noise reduction modes, RIT clear. Lots of simpler and cheaper than the KRC2, because it is just the buttons. (This might also be useful for the K3)
• Remote Tuning Knob -- borrow the idea from Ten-Tec. Perhaps part of the PFx module.
• Opto-encoder for RIT/XIT knob -- Replace the potentiometer with a optical encoder. That way, we wouldn't have to manually re-center it.

I intend to keep using my K2 for several years. It's my hope that Elecraft will be able to deliver some of these enhancements.

Cushcraft A3S/A743

As mentioned before, I've been using a Cushcraft A3S for several years now. I purchased it back in 1989, had it up for about six years, in the basement for five, loaned out for a couple of years to W1YM, and then back up since 2001 at my QTH.

There's many things to admire about the A3S. It's quite rugged, having survived several freak Georgia ice storms, but it only weighs 25 lbs even with the 40m option. While it's one of the last tribanders designed without computer modelling, it has reasonable performance on a small boom of 14 feet. It's been a popular product both in home installtions and even for Field Day. I have one ham buddy who used to use one on a pneumatic mast mounted to his conversion van.

Here are some of the things that I have learned about the A3S and A743:

• Tape the driven element insulator

A layer of electrical tape protects the driven element insulator from the Sun. There's not much of it exposed, but the sunlight will slowly break it down over time. Covered with tape, it will last forever. Use a sharp knife to open the U-bolt holes.

• Tape the trap end caps

The plastic trap end caps won't last forever. After about 5-8 years in the Sun, they will split. Cushcraft also thinks quite a lot of them -- replacing all 24 end caps cost me almost $50 in 2000. Try to preserve them by placing a single layer of tape around the circumference of the cap where it goes over the trap tube. The flat ends don't tend to split, since they aren't under any tension.

• Keep the trap holes down

While it is in the instruction manual, it's pretty easy to have an element section rotate and have one or more holes pointing anywhere but down. Leave them that way, and the traps will fill up with water quickly. Get the screw clamps on tight enough so the element sections don't rotate.

• Mount traps the correct way

The traps are designed so that the end connected to the cover tube is closest to the boom. Make sure all of them are this way. If you are not sure which way they are, you can carefully slide an end cap off to check. Mark the trap tube with an arrow using a Sharpie so you can remember. If you put the trap on backwards, it may be hard to diagnose.

• Use a balun

Cushcraft recommends making a balun by taking six loops of coax and taping them together. WA2SRQ wrote a posting to TowerTalk back in 1996 that showed that such a balun is much more effective when the turns are in a single layer and do not overlap or bunch. Six turns of coax on a 4 inch PVC form should work nicely. Ferrite beads a foot and a half long works well, too. I found a set of 10 of ferrite beads large enough to fit over RG-8 and wired them underneath the boom.

• Twist the element support rope

With the A743 option, the driven element droops far too much, so Cushcraft includes a non-conductive support rope and support mast. The instructions show the support rope should have a twist right at the top of the support mast. The twist prevents the rope from sliding back and forth through the plastic grommet as the driven element moves in the wind. It will eventually break. Don't ask me how I know -- just remember to include the twist.

• Element spacings

The A3S offers three choices of spacings along the elements, CW, MID and Phone. I took a tip from K7LXC. With the Cushcraft 40-2CD,

Steve recommends going half-way between the CW and MID settings.

The last time I had my A3S up, I used the Phone settings, and the SWR was very high at the bottom of the bands. So, I used values that were midway between CW and MID.

This results in a good compromise in covering the entire band. Here's the SWR curves through about 130 feet of RG-213.

Based on these graphs, I could probably shorten up the spacing between the 20m and 15m traps, and the 10m traps and the boom.

Regardless of the values you use, double check the measurements before putting the antenna up. When I first put up the A3S, the 10m director spacing was about six inches too short. You couldn' t tell from the SWR curves, but the antenna likely did not work as it should.

There may be better tribanders you can buy, but the Cushcraft A3S is a great performer. The A743 option adds 40m. At my QTH, this is my best 40m antenna, partly because it is mounted higher than anything else. Being able to rotate the pattern is often helpful in crowded contest conditions.